Glass in the form of powder (obtained by grinding glass frits) have been used for many years as sealants and as coating on glasses, ceramics, metals and in thick film microelectronics. To function as a low temperature sealant and as an overglaze in thick film microelectronics, the glass has to soften and flow at temperatures lower than 500° C. Most of the commercial compositions are based on lead borate (PbO—B2O3) system or its modifications with zinc (PbO—B2O3—ZnO). These glasses contain high levels of PbO (typically 70-80 wt %) and have low durability. Lead is toxic and the trend today is to limit its use and eventually eliminate it from compositions used in sealing and microelectronics. Most of the devices made with high lead containing glasses find their way to dump sites where the lead is leached into the water table, the leaching is facilitated because of the low durability of high lead containing glasses.
One object of the present invention was to devise lead-free glass compositions having a glass transition temperature (Tg) no higher than 450° C. and, in a form of powders, would demonstrate flow properties similar to those displayed by the above mentioned lead glasses.
Another object of the present invention is to provide glass compositions with sufficient durabilities, expansion coefficients and flow properties for sealing optical fibers to ceramic ferrules and Kovar™.
Yet another object of the present invention is to provide glass compositions, essentially lead-free, and having absorption bands in the visible and/or in the near infra red region of the spectrum to allow local heating (and hence sealing) by laser radiation.